Lead-based piezoelectric materials have been widely used in sensors, actuators, transducers, and many other electronic devices. The increasing applications of these devices generate a more aggravated environmental concern because these traditional lead-based piezoelectric ceramics typically contain more than 60% lead by weight. Intensive efforts have been made to develop lead-free piezoelectric materials to replace the lead-based compositions. Potassium sodium niobate (Group IA-VB) system is one of the most promising candidates as a lead-free piezoelectric ceramic material. Potassium sodium niobate, (K,Na)NbO3 (KNN) based ceramics have a broad operation temperature range due to high Curie temperature of about ˜420° C., and large piezoelectric coefficient in the bulk ceramic.
For the applications in various microelectronics and micro electromechanical devices and systems (MEMS), lead-free piezoelectric thin films instead of bulk ceramics are demanded. However, it is highly challenging to obtain KNN-based piezoelectric thin films with excellent piezoelectric performance properties. The piezoelectric coefficient for KNN thin films is typically far below the expected value as compared to the bulk counterparts. The effective d33 values for the KNN thin and thick films from a chemical solution deposition method are in the range of 40 to 61 pm/V. For KNN thin films prepared by a physical deposition method such as sputtering and pulsed laser deposition, the effective d33 is ˜50 pm/V. Under similar testing conditions, the effective d33 for PZT thin film with the constraint from a substrate could be in the range of 70 to 130 pm/V. Accordingly, it can be seen that the piezoelectric performance property of KNN thin films is substantially inferior to that of PZT thin films. There is therefore a need for an improvement on the piezoelectric performance of KNN thin films.
The high volatility of the alkali elements (including potassium and sodium) during thermal processing due to their high vapour pressure is a major reason for the difficulty in the control of the composition and obtaining excellent properties of the resulting KNN thin films. Evaporation of the alkali ions causes the formation of oxygen vacancies in the films, and hence large leakage and poor piezoelectric properties. One way is to add significant excess of potassium and/or sodium compositions in the precursor solution for the chemical solution approach or in the targets for sputtering or pulsed-laser deposition (PLD) method, aiming at compensation for the loss of these alkali ions during the thermal processing. Some improvement in the properties of the resulting KNN films is observed when excess potassium and/or sodium are introduced. However, this causes uncertainty and difficulty in controlling the compositional stoichiometry for the obtained KNN thin films and thus large leakage current still exists due to the uncontrolled volatilization of alkali ions, which limits the improvement of the piezoelectric properties.